Paper making machinery includes the use of felts to dewater paper sheets. In a paper making process, a slurry of paper making components is deposited on a fabric through which the liquid components are extracted to provide a continuous paper sheet. The paper sheet is transported on a continuous press felt to a pair of rollers where the sheet and felt pass between the nip of the rollers to dewater the paper sheet. The felts, generally composed of polyamides such as nylon, tend to accumulate microbiological deposits during use. The deposits eventually plug the channels within the felt which carry water away from the sheet resulting in less effective, nonuniform dewatering of the paper sheet.
Plugging of the channels of a felt causes a variety of paper defects including crushing, picking, poor drainage, sheet following, sheet pickup and blowing. A crush in the paper sheet occurs when water is pressed out of the channels at a higher rate than the sheet can allow. Plugging may also result in picking where the sheet fibers adhere to a felt as the sheet and felt are separated. A thin film of water can form between the sheet and felt such that the sheet follows the felt on the outgoing side of a press nip. Pickup problems associated with the transfer of a web of paper from a fourdrinier wire to a felt that carries the web to the presses are also influenced by plugging of the felt. The felt may also be subject to blowing, a localized accumulation of air from the felt which causes an air bubble between the sheet and the felt. Microbiological deposits can also form on a felt and plug it. The problems associated with felt plugging due to microbiological deposits can be alleviated by treating the felt with a biocide to remove or prevent the deposits.
Microbiological fouling of felts is commonly caused by aerobic and anaerobic bacteria, yeast, mold, blue green algae (cyanobacteria), green algae, diatoms, protozoa and the like. Species of bacteria and fungi which are commonly isolated from paper making felts include Chaetonium sp., Aspergillus niger, Penicillium sp. Trichoderma sp., Alternaria sp., Bacilus subtilis, Bacillus megatherium, Pseudomonas sp., Proteus sp., and Brevibacterium sp. Although these deposits can be removed from a fouled felt by treating the felt with a biocide, conventional means for identifying microbiological contamination cannot detect fouling in felts.
Microbiological contamination of paper is usually detected by spraying the paper with ninhydrin. The ninhydrin changes to a pink purple color upon reacting with degraded protein matter or amino acids. When ninhydrin is applied to a felt, however, it reacts with the nylon felt to form a blue color which obscures the pink purple color change.
The stain 2-(p-idiophenyl)-3-(p-nitrophenyl)-5-phenyl-tetrazolium chloride, hereinafter referred to as iodonitrotetrazolium, has been used extensively in histochemical research. The colorless compound forms a red formazan when it is reduced due to aerobic or anaerobic respiration. Iodonitrotetrazolium has been used to detect microorganisms in the water or pulp of a papermaking process by measuring UV absorption.
There is a need for an indicator which directly detects microbiological contamination on papermaking felts in order to identify fouling of the felt before the channels of the felt become clogged.